This invention relates to an electrical method and apparatus for providing indications of capacitance values, and particularly to such method and apparatus for measuring the ratio between the capacitances C.sub.1 and C.sub.2 of two capacitors.
As an example, it is known to measure pressure in a fluid byuusing a fixed capacitor and another capacitor subject to a variable pressure, and sensing changes in the ratio of the capacitances of the two capacitors. In other cases the two capacitances are arranged to change in opposite directions in response to a physical parameter such as pressure. There are other known situations as well in which it is desired to measure such ratio between the values of two capacitors. Typically the two capacitance values are nearly equal at some point in their expected range of variation in value, so that extraneous factors tending to affect the value of C.sub.1 /C.sub.2 will be to a great extent cancelled out when the ratio C.sub.1 /C.sub.2 is formed.
One general method wiich may be employed to measure C.sub.1 /C.sub.2 (or, equivalently, C.sub.2 /C.sub.1) is to place C.sub.1 and C.sub.2 alternately into a position in an oscillator circuit in which the capacitor affects the frequency of oscillation, preferably so that changes in the oscillation frequency are substantially inversely proportional to the value of the capacitance placed in the circuit. By using the same circuit to generate the oscillations for both capacitors, and by switching the capacitors in and out of the circuit frequently and rapidly, sources of error other than noise and drift of circuit component values are to some extent eliminated; by deriving a signal proportional to the ratio of the values C.sub.1 and C.sub.2, the effects of drifts in circuit parameters, such as resistances and amplifier gains, are reduced.
However, in such circuits there is always present some additional fixed capacitance, here designated C.sub.p, continuously present in the circuit and affecting the frequency of oscillation. C.sub.p determines the frequency of oscillations which occur in the absence of both C.sub.1 and C.sub.2, and as used herein includes not only stray circuit capacitances but also any fixed lumped capacitance continuously present in the circuit. Accordingly, the foregoing procedures serve not to measure C.sub.1 /C.sub.2, but instead measure C.sub.1 +C.sub.p /C.sub.2 +C.sub.p. The latter ratio is not in general the desired ratio C.sub.1 /C.sub.2, and inaccuracy therefore results, especially where the values of C.sub.1 and C.sub.2 are small. In addition, in such cases the ratio will have an unpredictable temperature coefficient of span or gage factor, due to the unpredictable temperature coefficient of C.sub.p.
It is therefore an object of the invention to provide a new and useful method and apparatus for providing electrical indications of the ratio between the capacitances C.sub.1 and C.sub.2 of a pair of capacitors.
Another object is to provide such method and apparatus which are accurate and reliable.
A further object is to provide such method and apparatus which are economical to make and use.